Steel wheels, aluminum wheels, and the like are used as motor vehicle wheels, to which tubes, tires, etc., are fitted. In particular, aluminum wheels are widely used since they have excellent properties in terms of light weight, anticorrosion, design, etc.
A multilayer coating film is usually formed on an aluminum substrate such as an aluminum wheel for the sake of protection and appearance. The formation of a multilayer coating film is conducted by, for example, a method of optionally coating the aluminum substrate with a primer, and then coating the substrate with a thermosetting, organic solvent-based, colored base coating composition, and thereafter with a thermosetting acrylic resin, organic solvent-based, clear coating composition.
Such a method of forming a multilayer coating film is problematic in that organic solvents of organic solvent-based coating compositions used in the method have harmful effects on the global environment.
Japanese Unexamined Patent Publication No. 1999-300271 discloses a method of forming a multilayer coating film using a powder coating composition that does not contain any organic solvent. In this method, a multilayer coating film is formed by coating a metal substrate with an epoxy resin powder coating composition, optionally followed by heating, and then coating the coated substrate with a powder coating composition such as a thermosetting acrylic resin powder coating composition, followed by heating. However, an improvement in the interlayer adhesion of the multilayer coating film is desired in this method. Furthermore, this method was not intended to provide a lustrous coating film.